In many uses of refrigerant compressors, it is desirable to be able to reduce the capacity or volume of displacement of the compressor under certain operating conditions in order to provide a cooling or heating rate more closely matching a heat load. A means intended to provide modulation or partial unloading of a rotary compressor is described in U.S. Pat. No. 3,767,328 as comprising a radially extending bore formed in a cylindrical wall member of the compressor and communicating with the cylinder and a passage in the wall member having a modulating port in a wall portion of the bore connecting a suction port to the bore and a plunger slidably mounted in the bore. The compressor is intended to operate at full capacity by introducing high pressure refrigerant into the bore behind the plunger to hold the plunger closed. When a reduced pressure is substituted, the plunger opens and gas compression in the cylinder is delayed until the bore is sealed by the rotor.
A plunger of this type has certain disadvantages. For full displacement operating conditions, the plunger is positioned in its fully extended position where it is in constant contact with the rotor completely filling the cylinder end of the bore. This increases the mechanical load of the compressor and causes a decrease in efficiency. For partial load operation, substantial reduction in compression capacity cannot be achieved, since the diameter of the bore cannot be made sufficiently large.